The explosive growth in mobile electronic devices such as smartphones and tablets creates an increasing need in the art for compact and efficient switching power converters so that users may recharge these devices. A flyback switching power converter is typically provided with a mobile device as its transformer provides safe isolation from AC household current. This isolation introduces a problem in that the power switching occurs at the primary side of the transformer but the load is on the secondary side. The power switching modulation for a flyback converter requires knowledge of the output voltage on the secondary side of the transformer. Such feedback can be obtained through opto-isolators bridging from the secondary side to the primary side but this adds to cost and control complexity. Thus, primary-only feedback techniques have been developed that use the reflected voltage on the primary side of the transformer in each switching cycle.
In a switching cycle for a flyback converter, the secondary current (the current in the secondary winding of the transformer) pulses high after the primary-side power switch is cycled off. The secondary current then ramps down to zero as power is delivered to the load. The delay between the power switch off time and the secondary current ramping to zero is denoted as the transformer reset time (Trst). The reflected voltage on the primary winding at the transformer reset time is proportional to the output voltage because there is no diode drop voltage on the secondary side as the secondary current has ceased flowing. The reflected voltage at the transformer reset time is thus directly proportional to the output voltage based upon the turn ratio in the transformer and other factors. Primary-only feedback techniques use this reflected voltage to efficiently modulate the power switching and thus modulate the output voltage.
The power switch in a flyback converter may comprise a transistor such as a MOSFET or a bipolar junction transistor (BJT). As compared to MOSFETs, BJTs are cheaper to manufacture. In addition, BJTs have less EMI noise and lessen the need for snubber circuitry. Thus, the use of BJT power switches in flyback converters has grown in popularity, particularly for low-power applications such as for the growing mobile device market. A particularly advantageous flyback converter having a BJT power switch is disclosed in commonly-assigned U.S. Pat. No. 8,289,732 (the '732 patent), the contents of which are incorporated by reference in their entirety. In this flyback converter architecture, the BJT collector current (and thus the primary current) is controlled on a pulse-by-pulse basis using primary-only feedback. A feedback sense voltage (Vsense) is sensed on the primary side (such as on an auxiliary winding) at the transformer reset time as discussed above. A controller for the flyback converter compares the feedback voltage Vsense to a reference voltage that represents the target voltage at the output (as scaled according to the turn ratio in the transformer) to generate an error signal. The controller processes the error signal to control the power switch accordingly so that the desired peak primary current is achieved in the next switching cycle. Each time the BJT power switch is switched on, the collector current linearly ramps from zero to the desired peak current for that pulse.
Although the resulting control architecture is remarkably low cost and efficient, the linear variation in the collector current for each current pulse complicates the formation of the appropriate base current. To function as a switch, the base current for the BJT power switch should be sufficiently greater than the ratio of its collector current and common emitter gain so that the BJT power switch is driven into saturation. The appropriate amount of base current thus varies according to the linear variation of the collector current. Driving this sufficient amount of base current to maintain the BJT power switch in saturation is thus not an easy task as a sufficient amount of overdrive is necessary for the peak primary/collector current that is achieved just prior to switching off the BJT power switch. One approach for ensuring that this peak collector current is achieved involves driving the base current according to the overdrive amount sufficient to achieve this peak collector current across the entire pulse. But this use of such a constant base current then wastes power due to the resulting excessive overdrive across the bulk of the pulse but for the peak collector current. An example of such constant base current driving is shown in FIG. 1. A base driver on signal pulses on and off to control a generation of constant base current (iB) pulses accordingly. During each base current pulse, the collector current (iC) ramps up from zero to a peak collector current value. This amount of base current is sufficient for overdrive at this peak collector current and is thus excessive during the remainder of each pulse.
Accordingly, there is a need in the art for improved base current driving techniques and systems for switching power converters having BJT power switches.